1. Field of the Invention
The present invention relates to handling (transport and storage) of objects during manufacture.
2. Description of the Prior Art
Efforts are being increasingly directed to automating the manufacture of all kinds of objects as completely as possible with a view to reducing production costs while achieving enhanced reliability of the products obtained.
Certain stages of manufacture have in fact been largely automated for the mass production of some types of objects. This is the case, for example, in operations involving assembly of simple component parts which are relatively sturdy and little affected by environmental conditions. Where parts of this type are concerned, the automation problems presented by operations involving cutting, bonding, welding, screwing and so on have been fairly satisfactorily solved.
In most installations, the objects being produced are transported from one workstation to another by a belt conveyor (traveling band) on which the objects rest. When an operation on a first object is completed at one workstation, the object is re-taken by the conveyor. The following object arrives and is thrust forward to the workstation in order to undergo the same treatment. In the meantime, the first object is transported by the conveyor to the following workstation a which it is subjected to another treatment operation without delay or after a waiting period which is as short as possible since the parts are processed in the order of arrival on the conveyor. If the individual workstations are correctly automated, it is possible to achieve a very high degree of automation. This presupposes, however,
that the objects to be processed are of rugged design, PA1 that they are capable of withstanding any impacts against each other when there is a waiting line at the input of a workstation, PA1 that they are capable of withstanding accelerations or decelerations caused by the often abrupt displacement of the conveyor as it moves in front of a stationary workstation and conversely, PA1 that all the objects are subjected to the same processing operations in the order of successive workstations along the conveyor, PA1 that the processing operations performed at each workstation have approximately identical time-durations. PA1 the objects to be handled are mechanically delicate or fragile, PA1 the individual manufacturing operations require long periods of time which are very different from each other, PA1 the operations do not necessarily follow in the same order for the different objects or batches of objects to be manufactured. PA1 conveying of objects between processing machines, PA1 introduction of objects into the machines and withdrawal of said objects therefrom, PA1 temporary storage of objects between two successive processing operations.
On the contrary, automation presents particularly difficult problems under circumstances in which:
An additional difficulty arises when the objects have to remain in a controlled clean atmosphere (in particular a dust-free environment), not only during successive processing operations but also between these operations.
The fabrication of integrated circuits accumulates these difficulties in its first production stages or in other words throughout all the processing steps carried out on semiconductor layers before they are cut into individual chips and encapsulated within a sealed package. It is for this reason that many difficulties are encountered when automating these initial processing steps whereas it proves much easier to control the automation of the following steps and in particular the assembly of an integrated circuit in a package.
However, automatic processing of semiconductor layers on an industrial production basis is a particularly desirable objective. Although it is true that, in practice, the atmosphere of production workshops is maintained in a highly controlled state of cleanliness, the coming and going of operating personnel represents an inevitable source of pollution which considerably reduces production yields.
Various specific systems for handling semiconductor wafers have been proposed. These systems are not built on the general principle of conventional belt conveyors for at least three reasons which arise from the foregoing remarks. In the first place, semiconductor wafers are too delicate. Secondly, processing times are too long and too variable from one processing step to the next (from several tens of minutes to several hours). Furthermore, a requirement to be met is that the semiconductor wafers should not all follow the same series of treatments. It is also desirable to ensure that the batches can subsequently return to a machine through which they have already passed. For example, it may be necessary to perform several ion implantations with other operations such as photolithography in between. If only one implanter is available, the semiconductor wafers have to be returned to the implanter several times, which is virtually impracticable on an industrial scale when using a conventional belt-conveyor assembly line.
Particular stress should be laid on the fact that, in processes for the fabrication of integrated circuits, a crucial problem is encountered in the intermediate storage of semiconductor wafers precisely by reason of the very long duration of certain operations. It is thus necessary to make provision for storage locations in which a batch of semiconductor wafers can await completion of processing of a preceding batch by a machine before being in turn brought to that machine. The storage operation is itself not readily compatible with the conventional arrangement of automated assembly lines, that is, with the use of belt conveyors for transporting objects in sequence from a first workstation to a final workstation.
It is for the reasons stated above that the handling systems proposed for taking into account the specific requirements of processing of integrated circuit wafers mostly have the following features in common: they make use of motor-driven carriages for transporting batches of wafers from one location to another. These carriages are guided mechanically by rails or electronically by optical or electromagnetic tracks marked-out on the plant-shop floor. Track switches are provided for ensuring that the carriages do not follow an unvarying closed-loop path but are capable on the contrary of moving towards either one machine or another, or else towards an intermediate storage magazine. It is essential to note that the carriages are all motor-driven in order to be self-propelled. An individual carriage is thus capable of moving from one machine to another and accurately stopping in front of a machine or a magazine for carrying out loading or unloading of semiconductor wafers without interfering with the movements of the other carriages.
Each motor-driven carriage accordingly has to be equipped with a sophisticated electronic system for the purpose of controlling their displacements (as a function of the batches being transported) and particularly of ensuring highly accurate control of stopping at the precise location at which each carriage can cooperate with a machine for effecting a transfer of semiconductor wafers. In some systems, the carriages become true autonomous robots.
Since the carriages are numerous, this accordingly gives rise to a major problem of reliability. Thus the occurrence of a failure in a carriage is liable to produce a considerable disturbance in the entire industrial process since the failure has the effect of stopping not only the carriage considered but also the other carriages which are moving on the same path.
Apart from the problem of reliability, the present Applicant considers that these handling systems suffer from a much more serious disadvantage in regard to the difficulty involved in maintaining a sufficiently clean ambient atmosphere around the semiconductor wafers when they are being transported by a carriage equipped with an electric motor. The motors have parts which are in frictional contact (for current collection) and thus produce metallic dust particles as well as sparks. On the whole, they are the cause o substantial pollution which it would be practically impossible to eliminate to a sufficient extent by any means other than enclosing the semiconductor wafers in air-tight boxes while they are being transported. By making use of air-tight boxes, however, the introduction of the semiconductor wafers within the processing machines would become a very complex procedure. Without such an air-tight box, the pollution produced mainly by the motors is too great, even if the wafers are maintained in a dedusted airstream (the heat of the motors would in any case perturb this airstream).
It is clearly apparent from all the foregoing considerations that very significant constraints of many different kinds make it difficult to achieve automation in the handling of certain products such as integrated circuit wafers while fabrication is in progress.
In order to provide the most satisfactory solution to these problems, the present invention proposes a handling installation which is organized in an entirely different manner to those proposed up to the present time.